Conventionally, there has been known a flow rate measuring device that is disposed in an intake passage through which an intake air flows. The flow rate measuring device includes a sensor chip (hereinafter, referred to as a “flow rate chip”) that measures a flow rate of the intake air and a sensor chip (hereinafter, referred to as a “humidity chip”) that measures a humidity of the intake air.
Such a flow rate measuring device includes a casing that holds the flow rate chip and the humidity chip and that protrudes inside of the intake passage. The humidity chip is held at a side wall of the casing (see, e.g., JP 2015-087196 A).
However, heat from the internal combustion engine is transferred to the humidity chip through the casing in the flow rate measuring device, and therefore a temperature of the humidity chip may increase. Thus, a temperature of a humidity detecting element, which is disposed on a surface of the humidity chip, may be also increased. As a result, the temperature of the humidity detecting element is different from an actual temperature of the intake air, which negatively effects on a detection value of the humidity of the intake air. For this reason, there is a need to prevent the heat transfer to the humidity chip from the casing.
For example, JP 5445535 B discloses an assembly to hold a humidity chip. The assembly is separately provided from the casing, and protrudes inside of the intake passage at a position away from the casing.
In such a flow rate measuring device, since the casing and the assembly are away from each other, heat transfer from the casing to the humidity chip is prohibited, and thus an increase in a temperature of the humidity chip can be suppressed. However, the assembly may transfer heat of the internal combustion engine to surrounding intake air, and as a result, a temperature of the intake air is increased, which may lead to an increase in a temperature of the humidity detecting element.
WO 2014/060161 A discloses a humidity chip covered by a cover.
In this measuring device, two small openings are defined in the cover to be arranged along a flow direction of an intake air. A small amount of an intake air is taken into the cover through the one opening at a downstream side, and then the intake air passes through the humidity chip. Eventually, the intake air is returned to the intake passage through the other opening at an upstream side.
Accordingly, the flow rate measuring device may provide function to prevent foreign material from flowing into the cover, thereby protecting the humidity chip from the foreign material. However, it is difficult for such a small amount of the intake air taken into the cover to sufficiently release heat. Therefore, heat is easily accumulated in the flow rate measuring device, and thus it is difficult to suppress an increase in a temperature of the humidity chip.
In view of the above, it is an objective of the present disclosure to provide a flow rate measuring device disposed in an intake passage, wherein an increase in a temperature of a humidity detecting element having a humidity chip can be suppressed.